Method of addressing messages and communications system

ABSTRACT

A method of establishing wireless communications between an interrogator and individual ones of multiple wireless identification devices, the method comprising utilizing a tree search method to attempt to identify individual ones of the multiple wireless identification devices so as to be able to perform communications, without collision, between the interrogator and individual ones of the multiple wireless identification devices, a search tree being defined for the tree search method, the tree having multiple nodes respectively representing subgroups of the multiple wireless identification devices, wherein the interrogator transmits a command at a node, requesting that devices within the subgroup represented by the node respond, wherein the interrogator determines if a collision occurs in response to the command and, if not, repeats the command at the same node. An interrogator configured to transmit a command at a node, requesting that devices within the subgroup represented by the node respond, the interrogator further being configured to determine if a collision occurs in response to the command and, if not, to repeat the command at the same node includes: receiving a first signal from an interrogator in accordance with an algorithm to identify a radio frequency identification (RFID) device in a field of the interrogator, the first signal comprising a first set of bits and requesting a response from one or more RFID devices in the field selected in accordance with at least the first set of bits; responsive to receiving the first signal, determining if the first set of bits is equal to a first portion of an identifier of the RFID device, and, if so, modulating a radio frequency (RF) field, provided by the interrogator, to communicate a reply to the interrogator in accordance with the algorithm and receiving, in accordance with the algorithm, a retransmission of the first signal from the interrogator in response to the interrogator receiving the reply without detecting a collision.

CROSS REFERENCE TO RELATED APPLICATION

This is a ContinuationMore than one reissue application has been filedfor the reissue of U.S. Pat. No. 6,282,186, which reissue applicationsare the initial reissue application Ser. No. 10/652,573, filed Aug. 28,2003 and now U.S. Pat. No. RE40,686, a continuation reissue applicationSer. No. 11/862,121, filed Sep. 26, 2007, a continuation reissueapplication Ser. No. 11/862,130, filed Sep. 26, 2007, a continuationreissue application Ser. No. 11/862,124, filed Sep. 26, 2007, and thepresent continuation reissue application, which is a continuationapplication of the reissue application Ser. No. 11/862,121, filed Sep.26, 2007, which is a continuation application of the reissue applicationSer. No. 10/652,573, filed Aug. 28, 2003, which is a reissue applicationof U.S. Pat. No. 6,282,186 having U.S. patent application Ser. No.09/556,235, which is a continuation application of U.S. patentapplication Ser. No. 09/026,050, filed Feb. 19, 1998, now U.S. Pat. No.6,061,344 and titled “Method of Addressing Messages and CommunicationsSystem”.

TECHNICAL FIELD

This invention relates to communications protocols and to digital datacommunications. Still more particularly, the invention relates to datacommunications protocols in mediums such as radio communication or thelike. The invention also relates to radio frequency identificationdevices for inventory control, object monitoring, determining theexistence, location or movement of objects, or for remote automatedpayment.

BACKGROUND OF THE INVENTION

Communications protocols are used in various applications. For example,communications protocols can be used in electronic identificationsystems. As large numbers of objects are moved in inventory, productmanufacturing, and merchandising operations, there is a continuouschallenge to accurately monitor the location and flow of objects.Additionally, there is a continuing goal to interrogate the location ofobjects in an inexpensive and streamlined manner. One way of trackingobjects is with an electronic identification system.

One presently available electronic identification system utilizes amagnetic coupling system. In some cases, an identification device may beprovided with a unique identification code in order to distinguishbetween a number of different devices. Typically, the devices areentirely passive (have no power supply), which results in a small andportable package. However, such identification systems are only capableof operation over a relatively short range, limited by the size of amagnetic field used to supply power to the devices and to communicatewith the devices.

Another wireless electronic identification system utilizes a large,board level, active transponder device affixed to an object to bemonitored which receives a signal from an interrogator. The devicereceives the signal, then generates and transmits a responsive signal.The interrogation signal and the responsive signal are typicallyradio-frequency (RF) signals produced by an RF transmitter circuit.Because active devices have their own power sources, and do not need tobe in close proximity to an interrogator or reader to receive power viamagnetic coupling. Therefore, active transponder devices tend to be moresuitable for applications requiring tracking of a tagged device that maynot be in close proximity to an interrogator. For example, activetransponder devices tend to be more suitable for inventory control ortracking.

Electronic identification systems can also be used for remote payment.For example, when a radio frequency identification device passes aninterrogator at a toll booth, the toll booth can determine the identityof the radio frequency identification device, and thus of the owner ofthe device, and debit an account held by the owner for payment of tollor can receive a credit card number against which the toll can becharged. Similarly, remote payment is possible for a variety of othergoods or services.

A communication system typically includes two transponders: a commanderstation or interrogator, and a responder station or transponder devicewhich replies to the interrogator.

If the interrogator has prior knowledge of the identification number ofa device which the interrogator is looking for, it can specify that aresponse is requested only from the device with that identificationnumber. Sometimes, such information is not available. For example, thereare occasions where the interrogator is attempting to determine which ofmultiple devices are within communication range.

When the interrogator sends a message to a transponder device requestinga reply, there is a possibility that multiple transponder devices willattempt to respond simultaneously, causing a collision, and thus causingan erroneous message to be received by the interrogator. For example, ifthe interrogator sends out a command requesting that all devices withina communications range identify themselves, and gets a large number ofsimultaneous replies, the interrogator may not be able to interpret anyof these replies. Thus, arbitration schemes are employed to permitcommunications free of collisions.

In one arbitration scheme or system, described in commonly assigned U.S.Pat. Nos. 5,627,544; 5,583,850; 5,500,650; and 5,365,551, all toSnodgrass et al. and all incorporated herein by reference, theinterrogator sends a command causing each device of a potentially largenumber of responding devices to select a random number from a knownrange and use it as that device's arbitration number. By transmittingrequests for identification to various subsets of the full range ofarbitration numbers, and checking for an error-free response, theinterrogator determines the arbitration number of every responderstation capable of communicating at the same time. Therefore, theinterrogator is able to conduct subsequent uninterrupted communicationwith devices, one at a time, by addressing only one device.

Another arbitration scheme is referred to as the Aloha or slotted Alohascheme. This scheme is discussed in various references relating tocommunications, such as Digital Communications: Fundamentals andApplication, Bernard Sklar, published January 1988 by Prentice Hall. Inthis type of scheme, a device will respond to an interrogator using oneof many time domain slots selected randomly by the device. A problemwith the Aloha scheme is that if there are many devices, or potentiallymany devices in the field (i.e. in communications range, capable ofresponding) then there must be many available slots or many collisionswill occur. Having many available slots slows down replies. If themagnitude of the number of devices in a field is unknown, then manyslots are needed. This results in the system slowing down significantlybecause the reply time equals the number of slots multiplied by the timeperiod required for one reply.

An electronic identification system which can be used as a radiofrequency identification device, arbitration schemes, and variousapplications for such devices are described in detail in commonlyassigned U.S. patent application Ser. No. 08/705,043, filed Aug. 29,1996, and Pat. No. 6,130,602, which is incorporated herein by reference.

SUMMARY OF THE INVENTION

The invention provides a wireless identification device configured toprovide a signal to identify the device in response to an interrogationsignal.

In one aspect, a method includes: receiving a first signal from aninterrogator in accordance with an algorithm to identify a radiofrequency identification (RFID) device in a field of the interrogator,the first signal comprising a first set of bits and requesting aresponse from one or more RFID devices in the field selected inaccordance with at least the first set of bits; responsive to receivingthe first signal, determining if the first set of bits is equal to afirst portion of an identifier of the RFID device, and, if so,modulating a radio frequency (RF) field, provided by the interrogator,to communicate a reply to the interrogator in accordance with thealgorithm; and receiving, in accordance with the algorithm, aretransmission of the first signal from the interrogator in response tothe interrogator receiving the reply without detecting a collision.

One aspect of the invention provides a method of establishing wirelesscommunications between an interrogator and individual ones of multiplewireless identification devices. The method comprises utilizing a treesearch method to attempt to identify individual ones of the multiplewireless identification devices so as to be able to performcommunications, without collision, between the interrogator andindividual ones of the multiple wireless identification devices. Asearch tree is defined for the tree search method. The tree has multiplenodes respectively representing subgroups of the multiple wirelessidentification devices. The interrogator transmits a command at a node,requesting that devices within the subgroup represented by the noderespond. The interrogator determines if a collision occurs in responseto the command and, if not, repeats the command at the same node.

Another aspect of the invention provides a communications systemcomprising an interrogator, and a plurality of wireless identificationdevices configured to communicate with the interrogator in a wirelessfashion. The interrogator is configured to employ tree searching toattempt to identify individual ones of the multiple wirelessidentification devices, so as to be able to perform communicationswithout collision, between the interrogator and individual ones of themultiple wireless identification devices. The interrogator is configuredto follow a search tree, the tree having multiple nodes respectivelyrepresenting subgroups of the multiple wireless identification devices.The interrogator is configured to transmit a command at a node,requesting that devices within the subgroup represented by the noderespond. The interrogator is further configured to determine if acollision occurs in response to the command and, if not, to repeat thecommand at the same node.

One aspect of the invention provides a radio frequency identificationdevice comprising an integrated circuit including a receiver, atransmitter, and a microprocessor. In one embodiment, the integratedcircuit is a monolithic single die single metal layer integrated circuitincluding the receiver, the transmitter, and the microprocessor. Thedevice of this embodiment includes an active transponder, instead of atransponder which relies on magnetic coupling for power and thereforehas a much greater range.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention are described below withreference to the following accompanying drawings.

FIG. 1 is a high level circuit schematic showing an interrogator and aradio frequency identification device embodying the invention.

FIG. 2 is a front view of a housing, in the form of a badge or card,supporting the circuit of FIG. 1 according to one embodiment theinvention.

FIG. 3 is a front view of a housing supporting the circuit of FIG. 1according to another embodiment of the invention.

FIG. 4 is a diagram illustrating a tree splitting sort method forestablishing communication with a radio frequency identification devicein a field of a plurality of such devices.

FIG. 5. is a diagram illustrating a modified tree splitting sort methodfor establishing communication with a radio frequency identificationdevice in a field of a plurality of such devices.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

This disclosure of the invention is submitted in furtherance of theconstitutional purposes of the U.S. Patent Laws “to promote the progressof science and useful arts” (Article 1, Section 8).

FIG. 1 illustrates a wireless identification device 12 in accordancewith one embodiment of the invention. In the illustrated embodiment, thewireless identification device is a radio frequency data communicationdevice 12, and includes RFID circuitry 16. The device 12 furtherincludes at least one antenna 14 connected to the circuitry 16 forwireless or radio frequency transmission and reception by the circuitry16. In the illustrated embodiment, the RFID circuitry is defined by anintegrated circuit as described in the above-incorporated patentapplication Ser. No. 08/705,043, filed Aug. 29, 1996 U.S. Pat. No.6,130,602. Other embodiments are possible. A power source or supply 18is connected to the integrated circuit 16 to supply power to theintegrated circuit 16. In one embodiment, the power source 18 comprisesa battery.

The device 12 transmits and receives radio frequency communications toand from an interrogator 26. An exemplary interrogator is described incommonly assigned U.S. patent application Ser. No. 08/907,689, filedAug. 8, 1997 Pat. No. 6,289,209 and incorporated herein by reference.Preferably, the interrogator 26 includes an antenna 28, as well asdedicated transmitting and receiving circuitry, similar to thatimplemented on the integrated circuit 16.

Generally, the interrogator 26 transmits an interrogation signal orcommand 27 via the antenna 28. The device 12 receives the incominginterrogation signal via its antenna 14. Upon receiving the signal 27,the device 12 responds by generating and transmitting a responsivesignal or reply 29. The responsive signal 29 typically includesinformation that uniquely identifies, or labels the particular device 12that is transmitting, so as to identify any object or person with whichthe device 12 is associated. Although only one device 12 is shown inFIG. 1, typically there will be multiple devices 12 that correspond withthe interrogator 26, and the particular devices 12 that are incommunication with the interrogator 26 will typically change over time.In the illustrated embodiment in FIG. 1, there is no communicationbetween multiple devices 12. Instead, the devices 12 respectivelycommunicate with the interrogator 26. Multiple devices 12 can be used inthe same field of an interrogator 26 (i.e., within communications rangeof an interrogator 26).

The radio frequency data communication device 12 can be included in anyappropriate housing or packaging. Various methods of manufacturinghousings are described in commonly assigned U.S. patent application Ser.No. 08/800,037, filed Feb. 13, 1997, and Pat. No. 5,988,510 which isincorporated herein by reference.

FIG. 2 shows but one embodiment in the form of a card or badge 19including a housing 11 of plastic or other suitable material supportingthe device 12 and the power supply 18. In one embodiment, the front faceof the badge has visual identification features such as graphics, text,information found on identification or credit cards, etc.

FIG. 3 illustrates but one alternative housing supporting the device 12.More particularly, FIG. 3 shows a miniature housing 20 encasing thedevice 12 and power supply 18 to define a tag which can be supported byan object (e.g., hung from an object, affixed to an object, etc.).Although two particular types of housings have been disclosed, otherforms of housings are employed in alternative embodiments.

If the power supply 18 is a battery, the battery can take any suitableform. Preferably, the battery type will be selected depending on weight,size, and life requirements for a particular application. In oneembodiment, the battery 18 is a thin profile button-type cell forming asmall, thin energy cell more commonly utilized in watches and smallelectronic devices requiring a thin profile. A conventional button-typecell has a pair of electrodes, an anode formed by one face and a cathodeformed by an opposite face. In an alternative embodiment, the powersource 18 comprises a series connected pair of button type cells. Inother alternative embodiments, other types of suitable power source areemployed.

The circuitry 16 further includes a backscatter transmitter and isconfigured to provide a responsive signal to the interrogator 26 byradio frequency. More particularly, the circuitry 16 includes atransmitter, a receiver, and memory such as is described in U.S. patentapplication Ser. No. 08/705,043 Pat. No. 6,130,602.

Radio frequency identification has emerged as a viable and affordablealternative to tagging or labeling small to large quantities of items.The interrogator 26 communicates with the devices 12 via anelectromagnetic link, such as via an RF link (e.g., at microwavefrequencies, in one embodiment), so all transmissions by theinterrogator 26 are heard simultaneously by all devices 12 within range.

If the interrogator 26 sends out a command requesting that all devices12 within range identify themselves, and gets a large number ofsimultaneous replies, the interrogator 26 may not be able to interpretany of these replies. Therefore, arbitration schemes are provided.

If the interrogator 26 has prior knowledge of the identification numberof a device 12 which the interrogator 26 is looking for, it can specifythat a response is requested only from the device 12 with thatidentification number. To target a command at a specific device 12,(i.e., to initiate point-on-point communication), the interrogator 26must send a number identifying a specific device 12 along with thecommand. At start-up, or in a new or changing environment, theseidentification numbers are not known by the interrogator 26. Therefore,the interrogator 26 must identify all devices 12 in the field (withincommunication range) such as by determining the identification numbersof the devices 12 in the field. After this is accomplished,point-to-point communication can proceed as desired by the interrogator26.

Generally speaking, RFID systems are a type of multiaccess communicationsystem. The distance between the interrogator 26 and devices 12 withinthe field is typically fairly short (e.g., several meters), so packettransmission time is determined primarily by packet size and baud rate.Propagation delays are negligible. In such systems, there is a potentialfor a large number of transmitting devices 12 and there is a need forthe interrogator 26 to work in a changing environment, where differentdevices 12 are swapped in and out frequently (e.g., as inventory isadded or removed). In such systems, the inventors have determined thatthe use of random access methods work effectively for contentionresolution (i.e., for dealing with collisions between devices 12attempting to respond to the interrogator 26 at the same time).

RFID systems have some characteristics that are different from othercommunications systems. For example, one characteristic of theillustrated RFID systems is that the devices 12 never communicatewithout being prompted by the interrogator 26. This is in contrast totypical multiaccess systems where the transmitting units operate moreindependently. In addition, contention for the communication medium isshort lived as compared to the ongoing nature of the problem in othermultiaccess systems. For example, in a RFID system, after the devices 12have been identified, the interrogator can communicate with them in apoint-to-point fashion. Thus, arbitration in a RFID system is atransient rather than steady-state phenomenon. Further, the capabilityof a device 12 is limited by practical restrictions on size, power, andcost. The lifetime of a device 12 can often be measured in terms ofnumber of transmissions before battery power is lost. Therefore, one ofthe most important measures of system performance in RFID arbitration istotal time required to arbitrate a set of devices 12. Another measure ispower consumed by the devices 12 during the process. This is in contrastto the measures of throughput and packet delay in other types ofmultiaccess systems.

FIG. 4 illustrates one arbitration scheme that can be employed forcommunication between the interrogator and devices 12. Generally, theinterrogator 26 sends a command causing each device 12 of a potentiallylarge number of responding devices 12 to select a random number from aknown range and use it as that device's arbitration number. Bytransmitting requests for identification to various subsets of the fullrange of arbitration numbers, and checking for an error-free response,the interrogator 26 determines the arbitration number of every responderstation capable of communicating at the same time. Therefore, theinterrogator 26 is able to conduct subsequent unterrupted communicationwith devices 12, one at a time, by addressing only one device 12.

Three variables are used: an arbitration value (AVALUE), an arbitrationmask (AMASK), and a random value ID (RV). The interrogator sends anIdentify command (IdentifyCmnd) causing each device of a potentiallylarge number of responding devices to select a random number from aknown range and use it as that device's arbitration number. Theinterrogator sends an arbitration value (AVALUE) and an arbitration mask(AMASK) to a set of devices 12. The receiving devices 12 evaluate thefollowing equation: (AMASK & AVALUE)==(AMASK & RV) wherein “&” is abitwise AND function, and wherein “==” is an equality function. If theequation evaluates to “1” (TRUE), then the device 12 will reply. If theequation evaluates to “0” (FALSE), then the device 12 will not reply. Byperforming this in a structured manner, with the number of bits in thearbitration mask being increased by one each time, eventually a device12 will respond with no collisions. Thus, a binary search treemethodology is employed.

An example using actual numbers will now be provided using only fourbits, for simplicity, reference being made to FIG. 4. In one embodiment,sixteen bits are used for AVALUE and AMASK. Other numbers of bits canalso be employed depending, for example, on the number of devices 12expected to be encountered in a particular application, on desired costpoints, etc.

Assume, for this example, that there are two devices 12 in the field,one with a random value (RV) of 1100 (binary), and another with a randomvalue (RV) of 1010 (binary). The interrogator is tying to establishcommunications without collisions being caused by the two devices 12attempting to communicate at the same time.

The interrogator sets AVALUE to 0000 (or “don't care” for all bits, asindicated by the character “X” in FIG. 4) and AMASK to 0000. Theinterrogator transmits a command to all devices 12 requesting that theyidentify themselves. Each of the devices 12 evaluate (AMASK &AVALUE)==(AMASK & RV) using the random value RV that the respectivedevices 12 selected. If the equation evaluates to “1” (TRUE), then thedevice 12 will reply. If the equation evaluates to “0” (FALSE), then thedevice 12 will not reply. In the first level of the illustrated tree,AMASK is 0000 and anything bitwise ANDed with all zeros results in allzeros, so both the devices 12 in the field respond, and there is acollision.

Next, the interrogator sets AMASK to 0001 and AVALUE to 0000 andtransmits an Identify command. Both devices 12 in the field have a zerofor their least significant bit, and (AMASK & AVALUE)==(AMASK & RV) willbe true for both devices 12. For the device 12 with a random value of1100, the left side of the equation is evaluated as follows (0001 &0000)=0000.

The right side is evaluated as (0001 & 1100)=0000. The left side equalsthe right side, so the equation is true for the device 12 with therandom value of 1100. For the device 12 with a random value of 1010, theleft side of the equation is evaluated as (0001 & 0000)=0000. The rightside is evaluated as (0001 & 1010)=0000. The left side equals the rightside, so the equation is true for the device 12 with the random value of1010. Because the equation is true for both devices 12 in the field,both devices 12 in the field respond, and there is another collision.

Recursively, the interrogator next sets AMASK to 0011 with AVALUE stillat 0000 and transmits an Identify command. (AMASK & AVALUE)==(AMASK &RV) is evaluated for both devices 12. For the device 12 with a randomvalue of 1100, the left side of the equation is evaluated as follows(0011 & 0000)=0000. The right side is evaluated as (0011 & 1100)=0000.The left side equals the right side, so the equation is true for thedevice 12 with the random value of 1100, so this device 12 responds. Forthe device 12 with a random value of 1010, the left side of the equationis evaluated as (0011 & 0000)=0000. The right side is evaluated as (0011& 1010)=0010. The left side does not equal the right side, so theequation is false for the device 12 with the random value of 1010, andthis device 12 does not respond; Therefore, there is no collision, andthe interrogator can determine the identity (e.g., an identificationnumber) for the device 12 that does respond.

De-recursion takes place, and the devices 12 to the right for the sameAMASK level are accessed when AVALUE is set at 0010, and AMASK is set to0011.

The device 12 with the random value of 1010 receives a command andevaluates the equation (AMASK & AVALUE)==(AMASK & RV). The left side ofthe equation is evaluated as (0011 & 0010)=0010. The right side of theequation is evaluated as (0011 & 1010)=0010. The right side equals theleft side, so the equation is true for the device 12 with the randomvalue of 1010. Because there are no other devices 12 in the subtree, agood reply is returned by the device 12 with the random value of 1010.There is no collision, and the interrogator 26 can determine theidentity (e.g., an identification number) for the device 12 that doesrespond.

By recursion, what is meant is that a function makes a call to itself.In other words, the function calls itself within the body of thefunction. After the called function returns, de-recursion takes placeand execution continues at the place just after the function call; i.e.at the beginning of the statement after the function call.

For instance, consider a function that has four statements (numbered1,2,3,4) in it, and the second statement is a recursive call. Assumethat the fourth statement is a return statement. The first time throughthe loop (iteration 1) the function executes the statement 2 and(because it is a recursive call) calls itself causing iteration 2 tooccur. When iteration 2 gets to statement 2, it calls itself makingiteration 3. During execution in iteration 3 of statement 1, assume thatthe function does a return. The information that was saved on the stackfrom iteration 2 is loaded and the function resumes execution atstatement 3 (in iteration 2), followed by the execution of statement 4which is also a return statement. Since there are no more statements inthe function, the function de-recurses to iteration 1. Iteration 1, hadpreviously recursively called itself in statement 2. Therefore, it nowexecutes statement 3 (in iteration 1). Following that it executes areturn at statement 4. Recursion is known in the art.

Consider the following code which can be used to implement operation ofthe method shown in FIG. 4 and described above.

-   Arbitrate(AMASK, AVALUE)    -   {        -   collision=IdentifyCmnd(AMASK, AVALUE) if (collision) then            -   {                -   /* recursive call for left side */ Arbitrate                    ((AMASK<<1)+1, AVALUE)                -   /* recursive call for right side */ Arbitrate                    ((AMASK<<1)+1, AVALUE+(AMASK+1))            -   } /* endif */    -   }/* return */

The symbol “<<” represents a bitwise left shift. “<<1” means shift leftby one place. Thus, 0001<<1 would be 0010. Note, however, that AMASK isoriginally called with a value of zero, and 0000<<1 is still 0000.Therefore, for the first recursive call, AMASK=(AMASK<<1)+1. So for thefirst recursive call, the value of AMASK is 0000+0001=0001. For thesecond call, AMASK=(0001<<)+1=0010+1=0011. For the third recursive call,AMASK=(0011<<1)+1=0110+1=0111.

The routine generates values for AMASK and AVALUE to be used by theinterrogator in an Identify command “IdentifyCmnd.” Note that theroutine calls itself if there is a collision. De-recursion occurs whenthere is no collision. AVALUE and AMASK would have values such as thefollowing assuming collisions take place all the way down to the bottomof the tree.

AVALUE AMASK 0000 0000 0000 0001 0000 0011 0000 0111 0000  1111* 1000 1111* 0100 0111 0100  1111* 1100  1111*

This sequence of AMASK, AVALUE binary numbers assumes that there arecollisions all the way down to the bottom of the tree, at which pointthe Identify command sent by the interrogator is finally successful sothat no collision occurs. Rows in the table for which the interrogatoris successful in receiving a reply without collision are marked with thesymbol “*”. Note that if the Identify command was successful at, forexample, the third line in the table then the interrogator would stopgoing down that branch of the tree and start down another, so thesequence would be as shown in the following table.

AVALUE AMASK 0000 0000 0000 0001 0000  0011* 0010 0011 . . . . . .

This method is referred to as a splitting method. It works by splittinggroups of colliding devices 12 into subsets that are resolved in turn.The splitting method can also be viewed as a type of tree search. Eachsplit moves the method one level deeper in the tree. Either depth-firstor breadth-first traversals of the tree can be employed. Depth firsttraversals are performed by using recursion, as is employed in the codelisted above. Breadth-first traversals are accomplished by using a queueinstead of recursion.

Either depth-first or breadth-first traversals of the tree can beemployed. Depth first traversals are performed by using recursion, as isemployed in the code listed above. Breadth-first traversals areaccomplished by using a queue instead of recursion. The following is anexample of code for performing a breadth-first traversal.

-   Arbitrate(AMASK, AVALUE)    -   {        -   enqueue(0,0)        -   while (queue !=empty)            -   (AMASK,AVALUE)=dequeue( )            -   collision=IdentifyCmnd(AMASK, AVALUE)            -   if (collision) then            -   {                -   TEMP=AMASK+1                -   NEW_AMASK=(AMASK<<1)+1                -   enqueue(NEW_AMASK, AVALUE)                -   enqueue(NEW_AMASK, AVALUE+TEMP)                -   } /* endif */    -   endwhile    -   }/* return */.

The symbol “!=” means not equal to. AVALUE and AMASK would have valuessuch as those indicated in the following table for such code.

AVALUE AMASK 0000 0000 0000 0001 0001 0001 0000 0011 0010 0011 0001 00110011 0011 0000 0111 0100 0111 . . . . . .

Rows in the table for which the interrogator is successful in receivinga reply without collision are marked with the symbol “*”.

FIG. 5 illustrates an embodiment wherein the interrogator 26 retries onthe same node that yielded a good reply. The search tree has a pluralityof nodes 51, 52, 53, 54 etc. at respective levels 32, 34, 36, 38, or 40.The size of subgroups of random values decrease in size by half witheach node descended.

The interrogator performs a tree search, either depth-first orbreadth-first in a manner such as that described in connection with FIG.4, except that if the interrogator determines that no collision occurredin response to an Identify command, the interrogator repeats the commandat the same node. This takes advantage of an inherent capability of thedevices, particularly if the devices use backscatter communication,called self-arbitration. Arbitration times can be reduced, and batterylife for the devices can be increased.

When a single reply is read by the interrogator, for example, in node52, the method described in connection with FIG. 4 would involveproceeding to node 53 and then sending another Identify command. Becausea device 12 in a field of devices 12 can override weaker devices, thisembodiment is modified such that the interrogator retries on the samenode 52 after silencing the device 12 that gave the good reply. Thus,after receiving a good reply from node 52, the interrogator remains onnode 52 and reissues the Identify command after silencing the devicethat first responded on node 52. Repeating the Identify command on thesame node often yields other good replies, thus taking advantage of thedevices natural ability to self-arbitrate.

AVALUE and AMASK would have values such as the following for adepth-first traversal in a situation similar to the one described abovein connection with FIG. 4.

AVALUE AMASK 0000 0000 0000 0001 0000 0011 0000 0111 0000  1111* 0000 1111* 1000  1111* 1000  1111* 0100 0111 0100  1111* 0100  1111* 1100 1111* 1100  1111*

Rows in the table for which the interrogator is successful in receivinga reply without collision are marked with the symbol “*”.

In operation, the interrogator transmits a command at a node, requestingthat devices within the subgroup represented by the node respond. Theinterrogator determines if a collision occurs in response to the commandand, if not, repeats the command at the same node.

In one alternative embodiment, the upper bound of the number of devicesin the field (the maximum possible number of devices that couldcommunicate with the interrogator) is determined, and the tree searchmethod is started at a level 32, 34, 36, 38, or 40 in the tree dependingon the determined upper bound. The level of the search tree on which tostart the tree search is selected based on the determined maximumpossible number of wireless identification devices that couldcommunicate with the interrogator. The tree search is started at a leveldetermined by taking the base two logarithm of the determined maximumpossible number. More particularly, the tree search is started at alevel determined by taking the base two logarithm of the power of twonearest the determined maximum possible number of devices 12. The levelof the tree containing all subgroups of random values is consideredlevel zero, and lower levels are numbered 1, 2, 3, 4, etc.consecutively.

Methods involving determining the upper bound on a set of devices andstarting at a level in the tree depending on the determined upper boundare described in a commonly assigned patent application (attorney docketMI40-118) U.S. Pat. No. 6,118,789, naming Clifton W. Wood, Jr. as aninventor, titled “Method of Addressing Messages and CommunicationsSystem,” filed concurrently herewith, and which is incorporated hereinby reference.

In one alternative embodiment, a method involving starting at a level inthe tree depending on a determined upper bound (such as the methoddescribed in the commonly assigned patent application mentioned above)is combined with a method comprising re-trying on the same node thatgave a good reply, such as the method shown and described in connectionwith FIG. 5.

Another arbitration method that can be employed is referred to as the“Aloha” method. In the Aloha method, every time a device 12 is involvedin a collision, it waits a random period of time before retransmitting.This method can be improved by dividing time into equally sized slotsand forcing transmissions to be aligned with one of these slots. This isreferred to as “slotted Aloha.” In operation, the interrogator asks alldevices 12 in the field to transmit their identification numbers in thenext time slot. If the response is garbled, the interrogator informs thedevices 12 that a collision has occurred, and the slotted Aloha schemeis put into action. This means that each device 12 in the field respondswithin an arbitrary slot determined by a randomly selected value. Inother words, in each successive time slot, the devices 12 decide totransmit their identification number with a certain probability.

The Aloha method is based on a system operated by the University ofHawaii. In 1971, the University of Hawaii began operation of a systemnamed Aloha. A communication satellite was used to interconnect severaluniversity computers by use of a random access protocol. The systemoperates as follows. Users or devices transmit at any time they desire.After transmitting, a user listens for an acknowledgment from thereceiver or interrogator. Transmissions from different users willsometimes overlap in time (collide), causing reception errors in thedata in each of the contending messages. The errors are detected by thereceiver, and the receiver sends a negative acknowledgment to the users.When a negative acknowledgment is received, the messages areretransmitted by the colliding users after a random delay. If thecolliding users attempted to retransmit without the random delay, theywould collide again. If the user does not receive either anacknowledgment or a negative acknowledgment within a certain amount oftime, the user “times out” and retransmits the message.

There is a scheme known as slotted Aloha which improves the Aloha schemeby requiring a small amount of coordination among stations. In theslotted Aloha scheme, a sequence of coordination pulses is broadcast toall stations (devices). As is the case with the pure Aloha scheme,packet lengths are constant. Messages are required to be sent in a slottime between synchronization pulses, and can be started only at thebeginning of a time slot. This reduces the rate of collisions becauseonly messages transmitted in the same slot can interfere with oneanother. The retransmission mode of the pure 11 Aloha scheme is modifiedfor slotted Aloha such that if a negative acknowledgment occurs, thedevice retransmits after a random delay of an integer number of slottimes.

Aloha methods are described in a commonly assigned patent application(attorney docket MI40-089) U.S. Pat. No. 6,275,476, naming Clifton W.Wood, Jr. as an inventor, titled “Method of Addressing Messages andCommunications System,” filed concurrently herewith, and which isincorporated herein by reference.

In one alternative embodiment, an Aloha method (such as the methoddescribed in the commonly assigned patent application mentioned above)is combined with a method involving re-trying on the same node that gavea good reply, such as the method shown and described in connection withFIG. 5.

In another embodiment, levels of the search tree are skipped. Skippinglevels in the tree, after a collision caused by multiple devices 12responding, reduces the number of subsequent collisions without addingsignificantly to the number of no replies. In real-time systems, it isdesirable to have quick arbitration sessions on a set of devices 12whose unique identification numbers are unknown. Level skipping reducesthe number of collisions, both reducing arbitration time and conservingbattery life on a set of devices 12. In one embodiment, every otherlevel is skipped. In alternative embodiments, more than one level isskipped each time.

The trade off that must be considered in determining how many (if any)levels to skip with each decent down the tree is as follows. Skippinglevels reduces the number of collisions, thus saving battery power inthe devices 12. Skipping deeper (skipping more than one level) furtherreduces the number of collisions. The more levels that are skipped, thegreater the reduction in collisions. However, skipping levels results inlonger search times because the number of queries (Identify commands)increases. The more levels that are skipped, the longer the searchtimes. Skipping just one level has an almost negligible effect on searchtime, but drastically reduces the number of collisions. If more than onelevel is skipped, search time increases substantially. Skipping everyother level drastically reduces the number of collisions and savesbattery power without significantly increasing the number of queries.

Level skipping methods are described in a commonly assigned patentapplication (attorney docket MI40-117) U.S. Pat. No. 6,072,801, namingClifton W. Wood, Jr. and Don Hush as inventors, titled “Method ofAddressing Messages, Method of Establishing Wireless Communications, andCommunications System,” filed concurrently herewith, and which isincorporated herein by reference.

In one alternative embodiment, a level skipping method is combined witha method involving re-trying on the same node that gave a good reply,such as the method shown and described in connection with FIG. 5.

In yet another alternative embodiment, any two or more of the methodsdescribed in the commonly assigned, concurrently filed, applicationsmentioned above are combined.

In compliance with the statute, the invention has been described inlanguage more or less specific as to structural and methodical features.It is to be understood, however, that the invention is not limited tothe specific features shown and described, since the means hereindisclosed comprise preferred forms of putting the invention into effect.The invention is, therefore, claimed in any of its forms ormodifications within the proper scope of the appended claimsappropriately interpreted in accordance with the doctrine ofequivalents.

1. A method of establishing wireless communications between aninterrogator and wireless identification devices, the method comprisingutilizing a tree search technique to establish communications, withoutcollision, between the interrogator and individual ones of the multiplewireless identification devices, the method including using a searchtree having multiple nodes respectively representing subgroups of themultiple wireless identification devices, the method further comprising,for a node, transmitting a command, using the interrogator, requestingthat devices within the subgroup represented by the node respond,determining with the interrogator if a collision occurred in response tothe command and, if not, repeating the command at the same node.
 2. Amethod in accordance with claim 1 and further comprising, if a collisionoccurred in response to the first mentioned command, sending a commandat a different node, using the interrogator.
 3. A method in accordancewith claim 1 wherein when a subgroup contains both a device that iswithin communications range of the interrogator, and a device that isnot within communications range of the interrogator, the device that isnot within communications range of the interrogator does not respond tothe command.
 4. A method in accordance with claim 1 wherein when asubgroup contains both a device that is within communications range ofthe interrogator, and a device that is not within communications rangeof the interrogator, the device that is within communications range ofthe interrogator responds to the command.
 5. A method in accordance withclaim 1 wherein a device in a subgroup changes between being withincommunications range of the interrogator and not being withincommunications range, over time.
 6. A method in accordance with claim 1wherein the wireless identification device comprises an integratedcircuit including a receiver, a modulator, and a microprocessor incommunication with the receiver and modulator.
 7. A method of addressingmessages from an interrogator to a selected one or more of a number ofcommunications devices, the method comprising: establishing forrespective devices unique identification numbers; causing the devices toselect random values, wherein respective devices choose random valuesindependently of random values selected by the other devices;transmitting a communication, from the interrogator, requesting deviceshaving random values within a first specified group of random values torespond; receiving the communication at multiple devices, devicesreceiving the communication respectively determining if the random valuechosen by the device falls within the first specified group and, if so,sending a reply to the interrogator; and determining using theinterrogator if a collision occurred between devices that sent a replyand, if so, creating a second specified group smaller than the firstspecified group; and, if not, again transmitting a communicationrequesting devices having random values within the first specified groupof random values to respond.
 8. A method of addressing messages from aninterrogator to a selected one or more of a number of communicationsdevices in accordance with claim 7 wherein sending a reply to theinterrogator comprises transmitting the unique identification number ofthe device sending the reply.
 9. A method in accordance with claim 7wherein one of the first and second specified groups contains both adevice that is within communications range of the interrogator, and adevice that is not within communications range of the interrogator, andwherein the device that is not within communications range of theinterrogator does not respond to the interrogator.
 10. A method ofaddressing messages from an interrogator to a selected one or more of anumber of communications devices in accordance with claim 7 wherein,after receiving a reply without collision from a device, theinterrogator sends a communication individually addressed to thatdevice.
 11. A method of addressing messages from a transponder to aselected one or more of a number of communications devices, the methodcomprising: establishing unique identification numbers for respectivedevices; causing the devices to select random values, wherein respectivedevices choose random values independently of random values selected bythe other devices; transmitting a communication from the transponderrequesting devices having random values within a specified group of aplurality of possible groups of random values to respond, the pluralityof possible groups being organized in a binary tree defined by aplurality of nodes at respective levels, the specified group beingdefined as being at one of the nodes; receiving the communication atmultiple devices, devices receiving the communication respectivelydetermining if the random value chosen by the device falls within thespecified group and, if so, sending a reply to the transponder; and, ifnot, not sending a reply; and determining using the transponder if acollision occurred between devices that sent a reply and, if so,creating a new, smaller, specified group by descending in the tree; and,if not, transmitting a communication at the same node.
 12. A method ofaddressing messages from a transponder to a selected one or more of anumber of communications devices in accordance with claim 11 whereinestablishing unique identification numbers for respective devicescomprises establishing a predetermined number of bits to be used for theunique identification numbers.
 13. A method of addressing messages froma transponder to a selected one or more of a number of communicationsdevices in accordance with claim 12 and further including establishing apredetermined number of bits to be used for the random values.
 14. Amethod of addressing messages from an interrogator to a selected one ormore of a number of RFID devices, the method comprising: establishingfor respective devices unique identification numbers; causing thedevices to select random values, wherein respective devices chooserandom values independently of random values selected by the otherdevices; transmitting a command using the interrogator requestingdevices having random values within a specified group of a plurality ofpossible groups of random values to respond, the specified group beingequal to or less than the entire set of random values, the plurality ofpossible groups being organized in a binary tree defined by a pluralityof nodes at respective levels; receiving the command at multiple RFIDdevices, RFID devices receiving the command respectively determining iftheir chosen random values fall within the specified group and, only ifso, sending a reply to the interrogator, wherein sending a reply to theinterrogator comprises transmitting the unique identification number ofthe device sending the reply; determining using the interrogator if acollision occurred between devices that sent a reply and, if so,creating a new, smaller, specified group using a different level of thetree, the interrogator transmitting a command requesting devices havingrandom values within the new specified group of random values torespond; and, if not, the interrogator re-transmitting a commandrequesting devices having random values within the first mentionedspecified group of random values to respond; and if a reply withoutcollision is received from a device, the interrogator subsequentlysending a command individually addressed to that device.
 15. A method ofaddressing messages from an interrogator to a selected one or more of anumber of RFID devices in accordance with claim 14 wherein the firstmentioned specified group contains both a device that is withincommunications range of the interrogator, and a device that is notwithin communications range of the interrogator, and wherein the devicethat is not within communications range of the interrogator does notrespond to the transmitting of the command or the re-transmitting of thecommand.
 16. A method of addressing messages from an interrogator to aselected one or more of a number of RFID devices in accordance withclaim 14 wherein the first mentioned specified group contains both adevice that is within communications range of the interrogator, and adevice that is not within communications range of the interrogator, andwherein the device that is within communications range of theinterrogator responds to the transmitting of the command and there-transmitting of the command.
 17. A method of addressing messages froman interrogator to a selected one or more of a number of RFID devices inaccordance with claim 14 wherein a device in the first mentionedspecified group is capable of changing between being withincommunications range of the interrogator and not being withincommunications range of the interrogator over time.
 18. A method ofaddressing messages from an interrogator to a selected one or more of anumber of RFID devices in accordance with claim 14 wherein the devicesrespectively comprise an integrated circuit including a receiver, amodulator, and a microprocessor in communication with the receiver andmodulator.
 19. A method of addressing messages from an interrogator to aselected one or more of a number of RFID devices in accordance withclaim 14 and further comprising, after the interrogator transmits acommand requesting devices having random values within the new specifiedgroup of random values to respond; devices receiving the commandrespectively determining if their chosen random values fall within thenew smaller specified group and, if so, sending a reply to theinterrogator.
 20. A method of addressing messages from an interrogatorto a selected one or more of a number of RFID devices in accordance withclaim 19 and further comprising, after the interrogator transmits acommand requesting devices having random values within the new specifiedgroup of random values to respond; determining if a collision occurredbetween devices that sent a reply and, if so, creating a new specifiedgroup and repeating the transmitting of the command requesting deviceshaving random values within a specified group of random values torespond using different specified groups until all of the devicescapable of communicating with the interrogator are identified.
 21. Acommunications system comprising an interrogator, and a plurality ofwireless identification devices configured to communicate with theinterrogator using RF, the interrogator being configured to employ treesearching to attempt to identify individual ones of the multiplewireless identification devices, so as to be able to performcommunications without collision between the interrogator and individualones of the multiple wireless identification devices, the interrogatorbeing configured to follow a search tree, the tree having multiple nodesrespectively representing subgroups of the multiple wirelessidentification devices, the interrogator being configured to transmit acommand at a node, requesting that devices within the subgrouprepresented by the node respond, the interrogator further beingconfigured to determine if a collision occurs in response to the commandand, if not, to repeat the command at the same node.
 22. Acommunications system in accordance with claim 21 wherein theinterrogator is configured to send a command at a different node if acollision occurs in response to the first mentioned command.
 23. Acommunications system in accordance with claim 21 wherein a subgroupcontains both a device that is within communications range of theinterrogator, and a device that is not within communications range ofthe interrogator.
 24. A communications system in accordance with claim21 wherein a subgroup contains both a device that is withincommunications range of the interrogator, and a device that is notwithin communications range of the interrogator, and wherein the devicethat is within communications range of the interrogator responds to thecommand.
 25. A communications system in accordance with claim 21 whereina device in a subgroup is movable relative to the interrogator so as tobe capable of changing between being within communications range of theinterrogator and not being within communications range.
 26. Acommunications system in accordance with claim 21 wherein the wirelessidentification device comprises an integrated circuit including areceiver, a modulator, and a microprocessor in communication with thereceiver and modulator.
 27. A system comprising: an interrogator; anumber of communications devices capable of wireless communications withthe interrogator; means for establishing for respective devices uniqueidentification numbers respectively having the first predeterminednumber of bits; means for causing the devices to select random values,wherein respective devices choose random values independently of randomvalues selected by the other devices; means for causing the interrogatorto transmit a command requesting devices having random values within aspecified group of random values to respond; means for causing devicesreceiving the command to determine if their chosen random values fallwithin the specified group and, if so, to send a reply to theinterrogator; and means for causing the interrogator to determine if acollision occurred between devices that sent a reply and, if so, tocreate a new, smaller, specified group; and, if not, transmit a commandrequesting devices having random values within the same specified groupof random values to respond.
 28. A system in accordance with claim 27wherein sending a reply to the interrogator comprises transmitting theunique identification number of the device sending the reply.
 29. Asystem in accordance with claim 27 wherein a specified group containsboth a device that is within communications range of the interrogator,and a device that is not within communications range of theinterrogator.
 30. A system in accordance with claim 27 wherein theinterrogator further includes means for, after receiving a reply withoutcollision from a device, sending a command individually addressed tothat device.
 31. A system comprising: an interrogator configured tocommunicate to a selected one or more of a number of communicationsdevices; and a plurality of communications devices; the devices beingconfigured to select random values, wherein respective devices chooserandom values independently of random values selected by the otherdevices; the interrogator being configured to transmit a commandrequesting devices having random values within a specified group of aplurality of possible groups of random values to respond, the specifiedgroup being less than the entire set of random values, the plurality ofpossible groups being organized in a binary tree defined by a pluralityof nodes at respective levels, the specified group being defined asbeing at one of the nodes; devices receiving the command beingconfigured to respectively determine if their chosen random values fallwithin the specified group and, only if so, send a reply to theinterrogator, wherein sending a reply to the interrogator comprisestransmitting the unique identification number of the device sending thereply; the interrogator being configured to determine if a collisionoccurred between devices that sent a reply and, if so, create a new,smaller, specified group using a different level of the tree, theinterrogator being configured to transmit a command requesting deviceshaving random values within the new specified group of random values torespond; and, if not, the interrogator being configured to re-transmit acommand requesting devices having random values within the firstmentioned specified group of random values to respond.
 32. A system inaccordance with claim 31 wherein the first mentioned specified groupcontains both a device that is within communications range of theinterrogator, and a device that is not within communications range ofthe interrogator.
 33. A system in accordance with claim 31 wherein adevice in the first mentioned specified group is capable of changingbetween being within communications range of the interrogator and notbeing within communications range of the interrogator over time.
 34. Asystem in accordance with claim 31 wherein the respective devicescomprise an integrated circuit including a receiver, a modulator, and amicroprocessor in communication with the receiver and modulator.
 35. Asystem comprising: an interrogator configured to communicate to aselected one or more of a number of RFID devices; a plurality of RFIDdevices, respective devices being configured to store a uniqueidentification number, respective devices being further configured tostore a random value; the interrogator being configured to transmit acommand requesting devices having random values within a specified groupof a plurality of possible groups of random values to respond, theplurality of possible groups being organized in a binary tree defined bya plurality of nodes at respective levels, the specified group beingdefined as being at one of the nodes; devices receiving the commandrespectively being configured to determine if their chosen random valuesfall within the specified group and, if so, send a reply to theinterrogator; and, if not, not send a reply; and the interrogator beingconfigured to determine if a collision occurred between devices thatsent a reply and, if so, to create a new, smaller, specified group bydescending in the tree; and, if not, to transmit a command at the samenode.
 36. A system in accordance with claim 35 wherein the uniqueidentification numbers for respective devices are stored in digital formand respectively comprise a predetermined number of bits.
 37. A systemin accordance with claim 35 wherein the random values for respectivedevices are stored in digital form and respectively comprise apredetermined number of bits.
 38. A system in accordance with claim 35wherein the interrogator is configured to determine if a collisionoccurred between devices that sent a reply in response to respectiveIdentify commands and, if so, to create further new specified groups andrepeat the transmitting of the command requesting devices having randomvalues within a specified group of random values to respond usingdifferent specified groups until all responding devices capable ofresponding are identified.
 39. A method, comprising: receiving a firstsignal from an interrogator in accordance with an algorithm to identifya radio frequency identification (RFID) device in a field of theinterrogator, the first signal having associated therewith a first setof bits and requesting a response from one or more RFID devices in thefield selected in accordance with at least the first set of bits;responsive to receiving the first signal, determining if the first setof bits matches a first portion of an identifier of the RFID device,and, if so, modulating a radio frequency (RF) field, provided by theinterrogator, to communicate a reply to the interrogator in accordancewith the algorithm; and receiving, in accordance with the algorithm,another transmission of the first signal from the interrogator inresponse to the interrogator receiving the reply without detecting acollision.
 40. The method of claim 39, further comprising communicatingwith the interrogator in one of a first communication mode and a secondcommunication mode determined by the interrogator, wherein in accordancewith the first communication mode the RFID device modulates a radiofrequency (RF) field generated by the RFID device and in accordance withthe second communication mode the RFID device modulates a radiofrequency (RF field generated by the interrogator.
 41. The method ofclaim 40, further comprising communicating with the interrogator at oneof a plurality of bit rates determined by the interrogator.
 42. Themethod of claim 39, further comprising receiving a wake up command fromthe interrogator and, in response, transitioning from a sleep state. 43.The method of claim 42, further comprising receiving a sleep commandfrom the interrogator.
 44. The method of claim 43, wherein the sleepcommand is received in response to the interrogator receiving the replywithout detecting a collision, in accordance with the algorithm, beforethe receiving of the retransmission of the first signal.
 45. The methodof claim 39, wherein the reply comprises a random value generated by theRFID device.
 46. The method of claim 45, wherein the identifiercomprises the random value.
 47. The method of claim 46, wherein therandom value is the identifier.
 48. The method of claim 39, furthercomprising: receiving, in accordance with the algorithm, a second signalfrom the interrogator in response to the interrogator detecting acollision in the reply, the second signal comprising a second set ofbits and requesting a response from one or more RFID devices in thefield selected in accordance with at least the second set of bits; andresponsive to receiving the second signal, determining if the second setof bits matches a second portion of the identifier of the RFID device,and, if so, modulating the radio frequency (RF) field to communicate asecond reply to the interrogator in accordance with the algorithm,wherein the second signal comprises the first signal, the second set ofbits comprises the first set of bits plus at least two additional bits,and the second portion of the identifier comprises the first portion ofthe identifier.
 49. The method of claim 48, wherein the second replycomprises at least a portion of the identifier.
 50. The method of claim49, further comprising communicating a random value to the interrogatorduring a time slot randomly selected from a number of time slots. 51.The method of claim 39, further comprising communicating a random valueto the interrogator during a time slot randomly selected from a numberof time slots.
 52. A system, comprising: a radio frequencyidentification (RFID) device comprising a receiver to receive a firstcommand including a portion of an identification number, memory to storean identifier of the device, and a transmitter to communicate a reply tothe first command if the portion of the identification number matches afirst portion of the identifier; and an interrogator configured toimplement an algorithm to identify one or more RFID devices in a fieldof the interrogator, the algorithm comprising transmitting a firstsignal with a first set of bits to request a response from a selectedone or more devices, receiving a first response thereto from theselected one or more devices, detecting if a collision occurred in thefirst response, and retransmitting the first signal with at least thefirst set of bits to request a second response from at least one of theselected one or more devices in response to detecting no collision inthe first response.
 53. The system of claim 52, further comprisingmemory storing a unique identification code to be transmitted by thesystem.
 54. The system of claim 52, wherein the transmitter isconfigured to communicate by modulating a radio frequency (RF) fieldprovided by a remote device.
 55. The system of claim 54, wherein thealgorithm further comprises transmitting an indication of the number ofbits of the first set of bits.
 56. The system of claim 52, whereinretransmitting the first signal with at least the first set of bitscomprises retransmitting the first signal with no more than the firstset of bits.
 57. The system of claim 52, wherein the system isconfigured to communicate at one of a plurality of bit rates determinedby a remote device.
 58. The system of claim 57, wherein the system isconfigured to operate in a first communication mode during a firstperiod of time and in a second communication mode during a second periodof time, wherein in accordance with the first communication mode thesystem is configured to modulate a radio frequency (RF) field generatedby the remote device and in accordance with the second communicationmode the system is configured to generate and modulate a radio frequency(RF) field.
 59. The system of claim 52, wherein the RFID device isconfigured to receive a signal to silence the RFID device.
 60. Thesystem of claim 59, wherein the RFID device is configured to receive awake up command and, in response, to transition from a sleep state. 61.The system of claim 52, wherein the algorithm further comprisestransmitting a signal to silence at least one of the one or more RFIDdevices in response to the detecting no collision and before theretransmitting of the first signal.
 62. The system of claim 52, whereinthe reply comprises at least a second portion of the identifier that isnot part of the first portion of the identifier.
 63. The system of claim62, wherein the interrogator is further configured to use the firstresponse to determine a random value generated by the selected one ormore devices in accordance with the algorithm.
 64. The system of claim52, wherein the algorithm further comprises transmitting a second signalfrom the interrogator in response to detecting a collision in the firstresponse, the second signal comprising a second set of bits andrequesting a response from at least one of the one or more RFID devicesin the field selected in accordance with at least the second set ofbits, wherein the second set of bits includes the first set of bits plusat least one additional bit.
 65. The system of claim 64, wherein theinterrogator is configured to generate, as part of the algorithm, thesecond set of bits including the first set of bits plus at least twoadditional bits.
 66. The system of claim 52, wherein the RFID device isconfigured to communicate a random value during a first time slotrandomly selected from a first number of timeslots.
 67. The system ofclaim 66, wherein the transmitter is configured to communicate bymodulating a radio frequency (RF) field provided by a remote device. 68.The system of claim 67, wherein the random value identifies the deviceto the remote device.
 69. The system of claim 68, wherein the RFIDdevice is further configured to communicate the random value to theremote device during a second time slot randomly selected from a secondnumber of time slots, wherein the first number of time slots isdifferent from the second number of time slots and is indicated by theremote device.
 70. The system of claim 69, wherein the algorithm furthercomprises transmitting a second signal from the interrogator in responseto detecting a collision in the first response, the second signalcomprising a second set of bits and requesting a response from at leastone of the one or more RFID devices in the field selected in accordancewith at least the second set of bits, wherein, in accordance with thealgorithm, the second set of bits include at least two bits in additionto the first set of bits.
 71. An apparatus for wirelessly reading atleast one radio frequency identification (RFID) device, comprising: atransmitter to transmit a command along with a first portion of a set ofidentifiers to request a response from at least one RFID device that hasan identifier in the set; an antenna to provide a radio frequency (RF)field to be modulated by the device; a receiver to receive the response;and processing circuitry to perform collision detection, to determinethe identifier using the response, and to cause the transmitter toretransmit the command along with at least the first portion of the setof identifiers responsive to detecting no collision in the response. 72.The apparatus of claim 71, wherein the transmitter is configured totransmit the command along with an indication of the number of bits ofthe first portion.
 73. The apparatus of claim 72, wherein thetransmitter is configured to communicate with the least one RFID deviceat one of a plurality of bit rates determined by the apparatus.
 74. Theapparatus of claim 71, wherein the processing circuitry is configured tocause the transmitter to retransmit the command along with no more thanthe first portion of the set of identifiers responsive to the detecting.75. The apparatus of claim 71, wherein the processing circuitry isconfigured to cause the transmitter to transmit a signal addressed tothe least one RFID device responsive to receiving the response withoutcollision.
 76. The apparatus of claim 75, wherein the signal isconfigured to place the least one RFID device in a listen-only mode. 77.The apparatus of claim 76, wherein the processing circuitry isconfigured to cause the transmitter to transmit the signal to place theleast one RFID device into a listen-only state before causing thetransmitter to retransmit the command.
 78. The apparatus of claim 71,wherein the transmitter is configured to transmit a wake up command totransition the least one RFID device from a sleep state.
 79. Theapparatus of claim 71, wherein the response comprises at least a secondportion of the identifier that is not part of the first portion.
 80. Theapparatus of claim 79, wherein the processing circuitry is configured todetermine a unique identification code stored in the least one RFIDdevice in addition to the identifier.
 81. The apparatus of claim 71,wherein the processing circuitry is configured to specify a secondportion of the set of identifiers in response to detecting a collision,the second portion being a subset of the first portion.
 82. Theapparatus of claim 81, wherein the processing circuitry is furtherconfigured to enable the second portion to be less than half of thefirst portion.
 83. The apparatus of claim 82, wherein the receiver is toreceive a reply from one or more RFID devices in one of a number of timeslots indicated by the apparatus to the one or more RFID devices. 84.The apparatus of claim 83, wherein the reply comprises a random valuegenerated by the one or more RFID devices.
 85. The apparatus of claim71, wherein the processing circuitry is further configured to cause thetransmitter to transmit a signal to indicate a number of time slots inwhich one or more RFID devices responds to the apparatus with a reply.86. The apparatus of claim 85, wherein the reply comprises a randomvalue generated by the one or more RFID devices.
 87. The apparatus ofclaim 86, wherein the one or more RFID devices comprises the RFID deviceand the random value matches the identifier value.
 88. The apparatus ofclaim 71, wherein the identifier identifies the device to the apparatus.89. The wireless apparatus of claim 71, wherein the wireless apparatusis at least partially disposed within a card.
 90. The wireless apparatusof claim 89, wherein the card is at least partially plastic.
 91. Thewireless apparatus of claim 89, wherein the card comprises a card usefulfor paying for goods or services.
 92. The wireless apparatus of claim71, the wireless apparatus is substantially disposed in a form factorsuitable for use within a vehicle, and is capable of interfacing with atoll booth.
 93. The wireless apparatus of claim 92, wherein the formfactor comprises a means for affixing the form factor to an object. 94.The method of claim 39, wherein the act communicating a reply to theinterrogator is performed as a part of a payment for goods or services.95. The method of claim 39, further comprising disposing the RFID deviceat least partially within a card.
 96. The method of claim 95, whereinthe card comprises a card useful for paying for goods or services. 97.The method of claim 95, further comprising disposing visualidentification information on a front side of the card, said visualidentification information identifying a financial entity.
 98. Themethod of claim 39, further comprising disposing RFID device in a formfactor suitable for use within a vehicle.
 99. The method of claim 39,wherein the form factor comprises a means for affixing the form factorto an object.
 100. The method of claim 39, wherein the communication ofa reply to the interrogator is performed as a part of a payment for atoll for use of a roadway.
 101. The method of claim 39, furthercomprising effecting an interface between the RFID device and a tollbooth.
 102. Wireless apparatus, comprising: first apparatus configuredto receive a command signal from an interrogator in accordance with analgorithm to identify a radio frequency identification (RFID) device ina field of the interrogator, the first command comprising a first set ofbits and requesting a response from one or more RFID devices in thefield selected in accordance with at least the first set of bits; andlogic configured to, responsive to receiving the first command,determine whether the first set of bits matches a first portion of anidentifier of the RFID device; and a second apparatus configured tomodulate a radio frequency (RF) field provided by the interrogator inorder to communicate a reply to the interrogator in accordance with thealgorithm; wherein said first apparatus is further configured to receivea second transmission of the first command from the interrogator inresponse to the interrogator receiving the reply without detecting acollision; and wherein said communication of the reply is performedbased at least in part on said determination of whether the first set ofbits matches a first portion of an identifier of the RFID device. 103.The wireless apparatus of claim 102, wherein the wireless apparatus isat least partially disposed within a card.
 104. The wireless apparatusof claim 103, wherein the card is at least partially plastic.
 105. Thewireless apparatus of claim 103, wherein the card comprises a paymentcard comprising visual identification information disposed on a frontside of the card.
 106. The wireless apparatus of claim 102, wherein thewireless apparatus is substantially disposed in a form factor suitablefor use within a vehicle, and is capable of interfacing with a tollbooth.
 107. The wireless apparatus of claim 106, wherein the form factorcomprises a means for affixing the form factor to an object.
 108. Thewireless apparatus of claim 102, wherein: the first apparatus is furtherconfigured to communicate with the interrogator in one of a firstcommunication mode and a second communication mode determined by theinterrogator; in accordance with the first communication mode, the RFIDdevice modulates a radio frequency (RF) field generated by the RFIDdevice; and in accordance with the second communication mode, the RFIDdevice modulates a radio frequency (RF) field generated by theinterrogator.
 109. The wireless apparatus of claim 108, wherein thesecond apparatus is further configured to communicate with theinterrogator at one of a plurality of bit rates determined by theinterrogator.
 110. The wireless apparatus of claim 102, wherein thefirst apparatus is further configured to receive a wake up command fromthe interrogator; and in response, to transition from a sleep state.111. The wireless apparatus of claim 110, wherein the first apparatus isfurther configured to receive a sleep command from the interrogator.112. The wireless apparatus of claim 111, wherein the sleep command isreceived in response to the interrogator receiving the reply withoutdetecting a collision, in accordance with the algorithm, before thereceiving of the retransmission of the first command.
 113. The wirelessapparatus of claim 102, wherein the reply comprises a random valuegenerated by the RFID device.
 114. The wireless apparatus of claim 113,wherein the identifier comprises the random value.
 115. The wirelessapparatus of claim 102, wherein the first apparatus is furtherconfigured to: receive, in accordance with the algorithm, a secondcommand from the interrogator in response to the interrogator detectinga collision in the reply, the second command comprising a second set ofbits and requesting a response from one or more RFID devices in thefield selected in accordance with at least the second set of bits; andresponsive to receive the second command, determine if the second set ofbits matches a second portion of the identifier of the RFID device, and,if so, modulate the radio frequency (RF) field to communicate a secondreply to the interrogator in accordance with the algorithm; wherein: thesecond command comprises the first command; the second set of bitscomprises the first set of bits plus at least two additional bits; andthe second portion of the identifier comprises the first portion of theidentifier.
 116. The wireless apparatus of claim 115, wherein the secondreply comprises at least a portion of the identifier.
 117. The wirelessapparatus of claim 102, wherein the second apparatus is furtherconfigured to communicate a random value to the interrogator during atime slot randomly selected from a number of time slots.
 118. A methodfor wirelessly reading at least one radio frequency identification(RFID) device, comprising: transmitting a command along with a firstportion of a set of identifiers to request a response from the at leastone RFID device that has an identifier in the set, the command beingtransmitted via an antenna configured to provide a radio frequency (RF)field to be modulated by the least one RFID device; receiving theresponse; and performing collision detection to determine the identifierusing the response, and to cause the transmitter to transmit the commanda second time along with at least the first portion of the set ofidentifiers responsive to detecting no collision.
 119. The method ofclaim 118, wherein the command transmission comprises an indication ofthe number of bits of the first portion.
 120. The method of claim 119,wherein the transmission is performed at one of a plurality of bitrates.
 121. The method of claim 118, further comprising transmitting thecommand said second time along with no more than the first portion ofthe set of identifiers responsive to the said detection of no collision.122. The method of claim 118, further comprising transmitting a signaladdressed to the at least one RFID device responsive to receiving theresponse to without detecting a collision.
 123. The method of claim 122,wherein the signal is configured to place the least one RFID device intoa listen-only state.
 124. The method of claim 122, wherein the act oftransmitting the signal to place the at least one RFID device into alisten-only state precedes the act of retransmitting of the command.125. The method of claim 118, wherein the response comprises at least asecond portion of the identifier that is not part of the first portion.126. The method of claim 125, further comprising determining a uniqueidentification code stored in the at least one RFID device in additionto the identifier.
 127. The method of claim 118, further comprisingspecifying a second portion of the set of identifiers in response todetecting a collision, the second portion being a subset of the firstportion.
 128. The method of claim 127, further comprising enabling thesecond portion to be less than half of the first portion.
 129. Themethod of claim 128, wherein the reply comprises a random valuegenerated by the one or more RFID devices.
 130. The method of claim 118,further comprising: providing to the at least one RFID devices a numberof time slots; causing the response from the at least one RFID devicewithin at least one of the number if time slots; and receiving theresponse in the at least one of the number of time slots.
 131. Themethod of claim 130, wherein the random value matches the identifiervalue.
 132. The method of claim 118, wherein the act wirelessly readingthe at least one RFID device is performed as part of a payment for goodsor services.
 133. The method of claim 118, further comprising disposingthe at least one RFID device at least partly within a card.
 134. Themethod of claim 133, wherein the card comprises a payment card usefulfor paying for goods or services.
 135. The method of claim 134, furthercomprising disposing visual identification information on a front sideof the card, said visual identification information identifying afinancial entity.
 136. The method of claim 118, further comprisingdisposing the at least one RFID device in a form factor suitable for usewithin a vehicle.
 137. The method of claim 136, wherein the form factorcomprises a means for affixing the form factor to an object.
 138. Themethod of claim 136, further comprising effecting an interface betweenthe at least one RFID device and a toll booth.
 139. The method of claim118, wherein the act of wirelessly reading the at least one RFID deviceis performed as a part of a payment for a toll for use of a roadway.140. A method of wireless communication of data, comprising: receiving afirst signal from an interrogator in accordance with an algorithm toidentify a radio frequency identification (RFID) device in a field ofthe interrogator, the first signal comprising a first set of bits andrequesting a response from the RFID device in the field selected inaccordance with at least the first set of bits; responsive to receivingthe first signal: determining if the first set of bits matches apredefined value related to the RFID device; and if so, modulating aradio frequency (RF) field, provided by the interrogator, to communicatea first reply to the interrogator in accordance with the algorithm; andreceiving, in accordance with the algorithm, a retransmission from theinterrogator of at least a portion of the first reply, in response tothe interrogator receiving the first reply without detecting acollision; wherein the first signal further comprises slot informationuseful for issuing said first reply by said RFID device.
 141. The methodof claim 140, wherein the act of wireless communication of data isperformed as a part of a payment for goods or services.
 142. The methodof claim 140, further comprising disposing the RFID device at leastpartly within a card.
 143. The method of claim 142, wherein the cardcomprises a payment card useful for paying for goods or services. 144.The method of claim 138, further comprising disposing visualidentification information on a front side of the card, said visualidentification information identifying a financial entity.
 145. Themethod of claim 140, further comprising disposing the one or more RFIDdevices in a form factor suitable for use within a vehicle.
 146. Theapparatus of claim 145, wherein the form factor comprises a means foraffixing the form factor to an object.
 147. The method of claim 146,further comprising effecting an interface between the RFID device and atoll booth.
 148. The method of claim 140, wherein the act of wirelesscommunication of data is performed as a part of a payment for a toll foruse of a roadway.
 149. The method of claim 140, further comprisingcommunicating with the interrogator in one of a first communication modeand a second communication mode determined by the interrogator; whereinin accordance with the first communication mode, the RFID devicemodulates a radio frequency (RF) field generated by the RFID device; andwherein in accordance with the second communication mode, the RFIDdevice modulates a radio frequency (RF) field generated by theinterrogator.
 150. The method of claim 140, further comprising:receiving a wake up command from the interrogator; and in response,transitioning from a sleep state.
 151. The method of claim 150, furthercomprising receiving a third signal from the interrogator, the thirdsignal configured to place the RFID device into a listen-only state.152. The method of claim 151, wherein the third signal is received inresponse to the interrogator receiving the reply without detecting acollision, before the receiving of the retransmission of the firstsignal.
 153. The method of claim 140, wherein the reply comprises arandom value generated by the RFID device.